In the past they let you upgrade your iOS but then you face performance degrades leading you to feel your device is old and you need to upgrade. With iOS 13, my older device runs just as fast or maybe faster than iOS 12 and maybe more stable.
In the past, Apple gave us, notably, Snow Leopard which just provided noticeably huge performance boost across the board. iOS 12 improved performance on the GPU by 20% and improved app launch performance compared to iOS 11
iOS 9 -> iOS 10 summary: "You won't lose performance, in fact you gain additional storage and faster closing of apps"
iOS 8 -> iOS 9: wifi and browsing performance improvements
iOS 7 vs 8 vs 9 vs 10 vs 11 on iPhone 5S. Your thesis should suggest that iOS 11 is slowest in all areas of tests, right? Turns out, iOS 11 is rarely last. Exception is in third party apps but that's because iOS 11 third party apps were running the latest versions while Snapchat on iOS 7 for example was running an older version (meaning less features and analytics running).
The fact that Apple has done things like Snow Leopard where performance was rock solid compared to previous versions, the fact that iOS 11 runs generally faster than iOS 8/9/10, the fact that iOS 12 runs faster than iOS 11 in some areas, and the fact that iOS 13 across the board runs faster than the last few versions highly suggests that there is no planned obsolescence.
If Apple really wanted to do planned obsolescence, they would simply stop supporting older hardware earlier. That is a less risky way of doing it. It's quite ridiculous to say that executives take the time to tell the engineers "hey, sprinkle some slower performing code here and there so that we can sell more devices" while every iOS release generally has huge bugs in them. Executives and engineers have better things to do than to figure how to intentionally slow devices down to sell more. They would rather spend time trying to stabilize the release so that every blog can stop writing at how bad these releases are getting.